Porous silicon relaxation medium for dislocation free CMOS devices

ABSTRACT

A method for forming CMOS devices includes masking a first portion of a tensile-strained silicon layer of a SOI substrate, doping a second portion of the layer outside the first portion and growing an undoped silicon layer on the doped portion and the first portion. The undoped silicon layer becomes tensile-strained. Strain in the undoped silicon layer over the doped portion is relaxed by converting the doped portion to a porous silicon to form a relaxed silicon layer. The porous silicon is converted to an oxide. A SiGe layer is grown and oxidized to convert the relaxed silicon layer to a compressed SiGe layer. Fins are etched in the first portion from the tensile-strained silicon layer and the undoped silicon layer and in the second portion from the compressed SiGe layer.

BACKGROUND Technical Field

The present invention relates to semiconductor processing, and moreparticularly to devices and methods for fabricating complementary metaloxide semiconductor (CMOS) devices having compressive and tensilestressed channel materials.

Description of the Related Art

Strain engineering is highly desired for boosting CMOS performance.Tensile strain is beneficial for N-type field effect transistors(NFETs), and compressive strain is beneficial for P-type field effecttransistors (PFETs). Conventional external strain techniques such as,embedded source/drain, stress liner, etc. begin to lose theireffectiveness due to highly scaled pitches and the three-dimensional(3D) nature of fin field effect transistors (finFETs).

Channel strain is one option that still works regardless of pitches anddevice architectures. However, forming a dually strained channel(tensile for NFET and compressive for PFET) is challenging.

SUMMARY

A method for forming complementary metal oxide semiconductor (CMOS)devices includes masking a first portion of a tensile-strained siliconlayer of a silicon-on-insulator substrate with a hard mask. A secondportion of the tensile-strained silicon layer is doped outside the firstportion. The hard mask is removed. An undoped silicon layer is grown onthe doped portion and the first portion, wherein the undoped siliconlayer becomes a tensile-strained undoped silicon layer. Strain isrelaxed in the undoped silicon layer over the doped portion byconverting the doped portion to porous silicon to form a relaxed siliconlayer. The porous silicon is converted to an oxide. A SiGe layer isgrown on the relaxed silicon layer. The SiGe layer is oxidized toconvert the relaxed silicon layer to a compressed SiGe layer. Fins areetched in the first portion from the tensile-strained silicon layer andthe undoped silicon layer and in the second portion from the compressedSiGe layer.

A method for forming complementary metal oxide semiconductor devicesincludes masking a first portion of a tensile-strained silicon layer ofa silicon on insulator substrate with a hard mask; etching a secondportion of the tensile-strained silicon layer outside the first portion;doping the second portion of the tensile-strained silicon layer outsidethe first portion; growing an undoped silicon layer on the dopedportion; removing the hard mask; relaxing strain in the undoped siliconlayer over the doped portion by converting the doped portion to a poroussilicon to form a relaxed silicon layer; converting the porous siliconto an oxide; growing a SiGe layer on the relaxed silicon layer;oxidizing the SiGe layer to convert the relaxed silicon layer to acompressed SiGe layer; etching fins in the first portion from thetensile-strained silicon layer and the undoped silicon layer and in thesecond portion from the compressed SiGe layer.

A complementary metal oxide semiconductor device includes a silicon oninsulator substrate, first fins formed in a first region from atensile-strained silicon layer on a buried dielectric layer of thesubstrate, a converted dielectric layer formed on the buried dielectriclayer of the substrate in a second region and second fins formed in thesecond region in a compression-strained layer on the converteddielectric layer. The first and second fins include different heights.The first fins are included in N-type field effect transistors, and thesecond fins are included in P-type field effect transistors.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional view of a silicon on insulator substrate(SOI) with a tensily strained layer in accordance with the presentprinciples;

FIG. 2 is a cross-sectional view of the substrate of FIG. 1 having ahard mask to protect an NFET side while doping the tensily strainedlayer on the PFET side in accordance with the present principles;

FIG. 3 is a cross-sectional view of the substrate of FIG. 2 having anundoped epitaxial layer formed on the tensily strained layer and dopedlayer on the PFET side in accordance with the present principles;

FIG. 4 is a cross-sectional view of the substrate of FIG. 3 having NFETand PFET regions defined by shallow trenches in accordance with thepresent principles;

FIG. 5 is a cross-sectional view of the substrate of FIG. 4 having thedoped layer converted to porous Si in accordance with the presentprinciples;

FIG. 6 is a cross-sectional view of the substrate of FIG. 5 having thedoped layer converted to oxide in accordance with the presentprinciples;

FIG. 7 is a cross-sectional view of the substrate of FIG. 6 havingshallow trenches filled in accordance with the present principles;

FIG. 8 is a cross-sectional view of the substrate of FIG. 7 having anepitaxially grow SiGe layer formed on a relaxed silicon layer inaccordance with the present principles;

FIG. 9 is a cross-sectional view of the substrate of FIG. 8 having theepitaxially grow SiGe layer condensed and oxidized to form a SiGe layerfrom the relaxed silicon layer in accordance with the presentprinciples;

FIG. 10 is a cross-sectional view of the substrate of FIG. 9 having NFETand PFET fins formed in accordance with the present principles;

FIG. 11 is a cross-sectional view of a silicon on insulator substrate(SOI) with a large tensily strained layer in accordance with anotherembodiment;

FIG. 12 is a cross-sectional view of the substrate of FIG. 11 having ahard mask to protect an NFET side while etching the tensily strainedlayer on the PFET side in accordance with the present principles;

FIG. 13 is a cross-sectional view of the substrate of FIG. 12 having adoped portion of the tensily strained layer on the PFET side and anundoped upper portion grown on the lower portion in accordance with thepresent principles;

FIG. 14 is a cross-sectional view of the substrate of FIG. 13 havingNFET and PFET regions defined by shallow trenches in accordance with thepresent principles;

FIG. 15 is a cross-sectional view of the substrate of FIG. 14 having thedoped layer converted to porous Si in accordance with the presentprinciples; and

FIG. 16 is a block/flow diagram showing a method for formingcomplementary metal oxide semiconductor devices in accordance with oneillustrative embodiment.

DETAILED DESCRIPTION

In accordance with the present principles, methods and structures areprovided for forming tensile strained NFETs and compressive strainedPFETs on a same chip. Both NFETs and PFETs have a dielectric layerformed under a channel to provide improved isolation that is better thanjunction isolation. In accordance with useful embodiments, a tensilystrained silicon-on-insulator (sSOI) has a hard mask formed thereon tocover an NFET region while a PFET region is exposed. In one embodiment,boron is doped in the sSOI in the PFET region followed by epitaxiallygrowing undoped Si on top of the boron-doped sSOI. NFET and PFET activeregions are then patterned. The boron-doped sSOI is converted intoporous Si so that the epitaxy Si relaxes. The porous Si is convertedinto an oxide to serve as an anchor for the relaxed Si. Then, SiGe isepitaxially grown on top of the relaxed Si to form compressivelystrained SiGe for the PFET. In this way, both tensile and compressiveregions (for NFETs and PFETs, respectively) are provided on a same chip

It is to be understood that the present invention will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps may be varied within the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

A design for an integrated circuit chip may be created in a graphicalcomputer programming language, and stored in a computer storage medium(such as a disk, tape, physical hard drive, or virtual hard drive suchas in a storage access network). If the designer does not fabricatechips or the photolithographic masks used to fabricate chips, thedesigner may transmit the resulting design by physical means (e.g., byproviding a copy of the storage medium storing the design) orelectronically (e.g., through the Internet) to such entities, directlyor indirectly. The stored design is then converted into the appropriateformat (e.g., GDSII) for the fabrication of photolithographic masks,which typically include multiple copies of the chip design in questionthat are to be formed on a wafer. The photolithographic masks areutilized to define areas of the wafer (and/or the layers thereon) to beetched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

It should be understood that material compounds will be described interms of listed elements, e.g., SiGe. These compounds include differentproportions of the elements within the compound, e.g., SiGe includesSi_(x)Ge_(1-x) (or Si_(1-x)Ge_(x)) where x is less than or equal to 1,etc. In addition, other elements may be included in the compound, andstill function in accordance with the present principles. The compoundswith additional elements will be referred to herein as alloys.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a silicon-on-insulator(SOI) substrate 10 or strained SOI (sSOI) includes a base 12, a burieddielectric layer 14 (e.g., a buried oxide or BOX) and tensily strainedsilicon layer 16. The base can be monocrystalline silicon,polycrystalline silicon, amorphous Si, and any other suitable materials.

Referring to FIG. 2, a hard mask 18 is deposited and patterned over thesilicon layer 16. The hard mask 18 is patterned to differentiate a PFETregion 24 and an NFET region 22 on the tensile-strained silicon layer 16such that the NFET region 22 is covered by the hard mask 18 and exposesthe layer 16 in the PFET region 24. The hard mask 18 may include siliconnitride (SiN) although other suitable materials may be employed.

Boron or other similar dopants are introduced into the tensile-strainedsilicon layer 16 in the PFET region 24. The doping process forintroducing dopants into a doped region 20 in the PFET region 24 mayinclude, e.g., solid phase doing, gas phase doping, or implantation.When needed, a dopant activation anneal (e.g., rapid thermal anneal,laser anneal, flash anneal) can be performed after the doping process.The doping concentration in the region 20 in the PFET region 24 mayrange from 3×10¹⁸/cm³ to 1×10²¹/cm³. More preferably, the dopingconcentration in the region 20 in the PFET region may range from1×10¹⁹/cm³ to 4×10²⁰/cm³.

Referring to FIG. 3, the hard mask 18 is removed and an undoped Si layer26 is epitaxially grown. The epitaxially grown Si layer 26 follows thelattice of the tensile-strained silicon layer 16 so the epitaxiallygrown Si layer 26 is also tensily strained. The epitaxially grownundoped Si layer 26 is formed over the tensile-strained silicon layer 16in both the NFET region 22 and the PFET region 24. Undoped means thedoping concentration is less than 1×10¹⁷/cm³.

Referring to FIG. 4, shallow trench isolation (STI) trenches 28 areetched into the layers 26 and 16 down to the buried dielectric layer 14.This patterns and defines active regions for NFETs and PFETs.

Referring to FIG. 5, the doped region 20 is converted into a porous Siregion 30 this in turn relaxes the epitaxially grown Si layer 26 overthe region 30 to provide a relaxed Si portion 32. The doped region 20can be converted into porous Si 30 by, e.g., an anodization process. Inthe PFET region 24, the originally tensile-strained silicon layer 16(now porous silicon region 30) is relaxed and the original layer 26 (nowlayer 32) on top of porous Si 30 also relaxes (porous Si has a very lowYoung's modulus). In the NFET region 22, the epitaxially grown undopedSi layer 26 follows lattice of the tensile-strained silicon layer 16 sothe is still the layer 26 remains tensily strained.

To show that the porous Si 30 also relaxes: let sSi=strained Si,pSi=porous Si, E is Young's modulus, E_(Si): ˜170 Gpa; E_(pSi) ˜17 GPawith ˜50% porosity. So E_(Si) ˜10 E_(pSi). Only 10% strain remains insSi, this means 90% relaxation is achieved.

Referring to FIG. 6, the porous Si 30 is converted into an oxide layer34 to serve as an anchor for the relaxed Si layer 32. A low temperatureoxidation (e.g., less than about 400 degrees C.) may be employed tooxidize the porous Si layer 30 to convert it into oxide. Porous Si (32)can be oxidized at a rate of about 100 times greater than non-porous Si.

Referring to FIG. 7, an STI fill process is performed to form STIregions 36. This may include depositing a dielectric material (e.g.,oxide) and then planarizing a top surface (e.g., by chemical mechanicalpolishing (CMP)).

Referring 8, process steps will be described now for formingcompressively strained SiGe in the PFET region 24. A hard mask 38, e.g.,formed from a nitride material or other suitable materials (e.g.,silicon oxide), is applied and patterned to cover the NFET region 22. ASiGe layer 42 is epitaxially grown on top of the relaxed Si layer 32.The SiGe layer 42 is compressively strained as formed. The epitaxiallygrown SiGe layer 42 is formed for the PFET region 24.

Referring to FIG. 9, a condensation process is employed to convert theSiGe layer 42 into an oxide layer 46 and turn the relaxed Si layer 32into a compressively strained SiGe (cSiGe) layer 44. The condensationprocess includes exposing the SiGe layer 42 to heat (e.g., 400-1050degrees C.) in the presence of oxygen to oxidize the Si in the SiGelayer to free up Ge. The Ge condenses out and is diffused into therelaxed Si layer 32 to form the compressive-strained SiGe layer 44.

Referring to FIG. 10, fins are formed for in both the NFET region 22 andthe PFET region 24. Fins 48 in the NFET region 22 are etched using areactive ion etch (RIE) process with a patterned mask (not shown). Thefins 48 are formed from tensile—strained Si. Fins 50 in the PFET region24 are etched using a reactive ion etch (RIE) process with a patternedmask (not shown). The fins 50 are formed from compressive—strained Si.The fins 48, 50 are employed to form complementary metal oxidesemiconductor (CMOS) devices, namely fin field effect transistors(finFETs). The finFETs include tensile NFETs, and compressive PFETs.

Tensile Si fins 48 and compressive SiGe fins 50 are formed differentthicknesses of dielectric. Fins 48 are formed on the buried dielectriclayer 14 and the fins 50 are formed on the buried dielectric layer 14and the oxide layer 34. The fins 48, 50 also provide different finheights (H_(Si)>H_(SiGe)). This enables a non-integer N/P ratio(NFET/PFET). The difference between NFET and PFET fin heights can bedetermined by the starting thickness of the silicon layer 16. Inaddition, all fin tops are provided at a same level 52. Therefore, noadditional process complexity is added in accordance with the presentprinciples.

Referring to FIG. 11, a silicon-on-insulator (SOI) substrate 10′ orstrained SOI (sSOI) includes a monocrystalline silicon base 12, a burieddielectric layer 14 (e.g., a buried oxide or BOX) and tensily strainedsilicon layer 16′ that is sized in accordance with fins to be formed.

Referring to FIG. 12, a hard mask 18 is deposited and patterned over thesilicon layer 16′. The hard mask 18 is patterned to differentiate anNFET region 22 and an PFET region 24 on the tensile-strained siliconlayer 16′ such that the NFET region 22 is covered by the hard mask 18and exposes the layer 16′ in the PFET region 24. The hard mask 18 mayinclude silicon nitride although other suitable materials may beemployed. An etch process is performed to form a recess 60 in the layer16′ in the PFET region 24.

Referring to FIG. 13, boron or other similar dopants are introduced intothe tensile-strained silicon layer 16′ in the PFET region 24. The dopingprocess for introducing dopants into a doped region 62 in the PFETregion 24 may include, e.g., solid phase doing, gas phase doping, orimplantation. An undoped Si layer 64 is epitaxially grown on the dopedregion 62. Both the doping and the epitaxial growth can be done togetherin an epitaxial tool or the steps may be performed separately indifferent tools or at different times.

Referring to FIG. 14, the hard mask 18 is removed. Shallow trenchisolation (STI) trenches 66 are etched into the layers 26 and 16 down tothe buried dielectric layer 14. This patterns and defines active regionsfor NFETs and PFETs.

Referring to FIG. 15, the doped region 62 is converted into porous Silayer 68 (e.g., by anodization). In the PFET region 24, the undoped Silayer 64 on top of the porous Si layer 68 relaxes. In the NFET region22, the Si layer 16′ is still tensily strained. The process continues asdescribed above.

It should be understood that the actual remaining strain in sSi afterporous Si patterning can be further engineered base on a starting sSithickness, porous Si thickness, the amount of porosity in the porous Siand other factors. As long as the majority (e.g., at least about 50-60%or more) of the strain in the sSi is relaxed in PFET region 24, theepitaxial SiGe layer 42 on top of the relaxed Si will be compressivelystrained. There is no need for 100% relaxation of sSi (of course, 100%relaxation is better than 90%).

Referring to FIG. 16, methods for forming complementary metal oxidesemiconductor devices are described in accordance with illustrativeembodiments. In some alternative implementations, the functions noted inthe blocks may occur out of the order noted in the figures. For example,two blocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts or carry outcombinations of special purpose hardware and computer instructions.

In one embodiment, in block 102, a first portion of a tensile-strainedsilicon layer of a silicon on insulator substrate is masked. The maskmay include a SiN hard mask patterned using lithography or otherprocess. In block 104, a second portion of the tensile-strained siliconlayer is doped outside the first portion. The doping may include borondoing although other dopants may be employed. The doping may includesolid phase, gas phase or implant doping.

In block 106, the hard mask is removed. In block 108, an undoped siliconlayer is grown on the doped portion and the first portion, wherein theundoped silicon layer becomes a tensile-strained undoped silicon layer.

In an alternate embodiment, in block 140, a second portion of thetensile-strained silicon layer is etched outside the first portion usinga hard mask. The tensile-strained silicon layer includes a thickness hasthe thickness of both the strain silicon layer and the undoped siliconlayer described in blocks 102 and 108. In block 142, the second portionof the tensile-strained silicon layer is doped outside the first portion(e.g., with boron). In block 144, an undoped silicon layer is grown onlyon the doped portion. In block 146, the hard mask is removed.

Both of the above methods continue as follows: In block 110, shallowtrench isolation (STI) trenches may be formed at least between the firstportion and the doped portion. In block 112, strain in the undopedsilicon layer over the doped portion is relaxed by converting the dopedportion to a porous silicon to form a relaxed silicon layer. The dopedportion may be converted to the porous silicon having or a porosity ofat least 50%. Other porosities may also be employed. The strain isrelaxed strain by at least 50%, greater relaxation percentages arepreferable (e.g., up to and including 100%).

In block 116, the porous silicon is converted to an oxide. This mayinclude anodizing the doped portion. In block 118, the STI trenches maybe filled. In block 120, a SiGe layer is grown on the relaxed siliconlayer. In block 122, the SiGe layer is oxidized to convert the relaxedsilicon layer to a compressed SiGe layer. A condensation process may beemployed to form the compressed SiGe layer. In one embodiment, thecondensation process includes a temperature of between about 400 degreesC. and 1050 degrees for one or more cycles (or less than a minute each)in the presence of oxygen.

In block 124, fins are etched in the first portion from thetensile-strained silicon layer and the undoped silicon layer, and in thesecond portion from the compressed SiGe layer. In block 126, N-typefield effect transistors are formed from the fins formed from thetensile-strained silicon layer and the tensile-strained silicon layer.P-type field effect transistors are formed from the compressed SiGelayer. In block 128, the N-type field effect transistors and the P-typefield effect transistors have different heights and the heights areadjusted to adjust N/P ratio. The heights can be adjusted based on astarting height of the silicon layers, porosity of the porous silicon,the porous silicon thickness, etc. In block 130, processing continues tocomplete the device.

Having described preferred embodiments for porous silicon relaxationmedium for dislocation free CMOS devices (which are intended to beillustrative and not limiting), it is noted that modifications andvariations can be made by persons skilled in the art in light of theabove teachings. It is therefore to be understood that changes may bemade in the particular embodiments disclosed which are within the scopeof the invention as outlined by the appended claims. Having thusdescribed aspects of the invention, with the details and particularityrequired by the patent laws, what is claimed and desired protected byLetters Patent is set forth in the appended claims.

The invention claimed is:
 1. A method for forming complementary metaloxide semiconductor devices, the method comprising: doping a secondportion of a tensile-strained silicon layer outside a first portion toform a doped portion; growing an undoped silicon layer on the dopedportion; and relaxing strain in the undoped silicon layer over the dopedportion by converting the doped portion to an oxide to form a relaxedlayer.
 2. The method as recited in claim 1, further comprising growing aSiGe layer on the relaxed layer.
 3. The method as recited in claim 2,further comprising oxidizing the SiGe layer to convert the relaxed layerto a compressed SiGe layer.
 4. The method as recited in claim 3, furthercomprising etching fins from the tensile-strained silicon layer and thecompressed SiGe layer.
 5. The method as recited in claim 4, whereindoping the second portion of the tensile-strained silicon layer includesboron doping the second portion.
 6. The method as recited in claim 5,wherein converting the doped portion to an oxide to form a relaxed layerincludes converting the doped portion to a porous silicon thenconverting the porous silicon to the oxide.
 7. The method as recited inclaim 6, wherein converting the doped portion to a porous siliconincludes converting the doped portion to the porous silicon with aporosity of at least 50%.
 8. The method as recited in claim 7, whereinoxidizing the SiGe layer to convert the relaxed silicon layer to acompressed SiGe layer includes employing a condensation process to formthe compressed SiGe layer.
 9. The method as recited in claim 8, furthercomprising forming N-type field effect transistors from the fins formedfrom the tensile-strained silicon layer.
 10. The method as recited inclaim 9, further comprising forming P-type field effect transistors fromthe fins formed from the compressed SiGe layer.
 11. The method asrecited in claim 10, wherein the N-type field effect transistors and theP-type field effect transistors have different heights and the methodfurther comprises adjusting the heights to adjust N/P ratio.
 12. Themethod as recited in claim 11, wherein relaxing strain in the undopedsilicon layer includes relaxing strain by at least 50%.
 13. The methodas recited in claim 12, wherein converting the doped portion to a poroussilicon to form a relaxed silicon layer includes anodizing the dopedportion.
 14. A method for forming complementary metal oxidesemiconductor devices, the method comprising: etching a second portionof a tensile-strained silicon layer outside a first portion using a hardmask; doping the second portion of the tensile-strained silicon layeroutside the first portion; growing an undoped silicon layer on the dopedportion; and relaxing strain in the undoped silicon layer over the dopedportion by converting the doped portion to an oxide to form a relaxedlayer.
 15. The method as recited in claim 14, further comprising growinga SiGe layer on the relaxed layer.
 16. The method as recited in claim15, further comprising oxidizing the SiGe layer to convert the relaxedlayer to a compressed SiGe layer.
 17. The method as recited in claim 16,further comprising etching fins from the tensile-strained silicon layerand the compressed SiGe layer.
 18. The method as recited in claim 17,wherein doping the second portion of the tensile-strained silicon layerincludes boron doping the second portion.
 19. The method as recited inclaim 18, wherein converting the doped portion to an oxide to form arelaxed layer includes converting the doped portion to a porous siliconthen converting the porous silicon to the oxide.
 20. The method asrecited in claim 19, wherein converting the doped portion to a poroussilicon includes converting the doped portion to the porous silicon witha porosity of at least 50%.